Aims. Magnification and de-magnification
due to gravitational lensing will contribute to the
brightness scatter of type Ia supernovae (SNe Ia). The purpose of
this paper is to investigate the possibility of decreasing this
scatter by correcting individual SNe Ia
using observations of galaxies in the foreground,
without introducing any extra bias.

Methods. We simulate a large number of SN Ia lines of sight populated by galaxies.
For each line of sight the true magnification factor and
an estimate thereof are calculated.
The estimated magnification factor corresponds to what an
observer would infer from a survey like the SNLS. Using the simulated
data we investigate the possibility of estimating the magnification of
individual supernovae with enough precision to be able to
correct their brightness for gravitational lensing with negligible bias.

Results.
Our simulations show that the bias arising from gravitational lensing
corrections of individual SNe Ia is negligible for current and next
generation surveys and that the scatter from lensing can be
reduced by approximately a factor of 2.
The total scatter in the SN Ia magnitudes could be reduced by 4% for an intrinsic dispersion
of 0.13 mag. For an intrinsic dispersion of 0.09 mag,
which may be feasible for future surveys, the total scatter could be reduced by 6%.
This will reduce the errors on cosmological parameters derived from supernova data
by 4–8%. The prospect of correcting for lensing is thus very good.

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